This study quantified Cd, Pb, and Cu content, and
the soil–plant transfer factors of these elements in rice paddies
within Cam Pha, Quang Ninh province, northeastern Vietnam.
The rice paddies are located at a distance of 2 km from the
large Coc Sau open-pit coal mine. Electron microprobe analysis
combined with backscattered electron imaging and
energy-dispersive spectroscopy revealed a relatively high proportion
of carbon particles rimmed by an iron sulfide mineral
(probably pyrite) in the quartz–clay matrix of rice paddy soils
at 20–30 cm depth. Bulk chemical analysis of these soils
revealed the presence of Cd, Cu, and Pb at concentrations of
0.146±0.004, 23.3±0.1, and 23.5±0.1 mg/kg which
exceeded calculated background concentrations of 0.006±
0.004, 1.9±0.5, and 2.4±1.5 mg/kg respectively at one of
the sites. Metals and metalloids in Cam Pha rice paddy soils,
including As, Cd, Cr, Cu, Hg, Mn, Ni, Pb, and Zn, were found
in concentrations ranging from 0.2±0.1 to 140±3 mg/kg,
which were in close agreement with toxic metal contents in
mine tailings and Coc Sau coal samples, suggesting mining
operations as a major cause of paddy soil contamination.
Native and model Oryza sativa L. rice plants were grown in
the laboratory in a growth medium to which up to 1.5 mg/kg
of paddy soil from Cam Pha was added to investigate the
effects on plant growth. A decrease in growth by up to 60 %
with respect to a control sample was found for model plants,
whereas a decrease of only 10 % was observed for native (Nep
cai hoa vang variety) rice plants. This result suggests an
adaptation of native Cam Pha rice plants to toxic metals in
the agricultural lands. The Cd, Cu, and Pb contents of the
native rice plants from Cam Pha paddies exceeded permitted
levels in foods. Cadmium and Pb were highest in the rice plant
roots with concentrations of 0.84±0.02 and 7.7±0.3 mg/kg,
suggesting an intake of these metals into the rice plant as
shown, for example, by Cd and Pb concentrations of 0.09±
0.01 and 0.10±0.04 mg/kg respectively in the rice grain
endosperm. The adaptation of native rice plants, combined
with bioaccumulation ratios of 1±0.6 to 1.4±0.7 calculated
for Cd transfer to the rice grain endosperm, and maximum Cd
transfer factors of 4.3±2.1 to the plant roots, strongly suggest
a continuous input of some toxic metals from coal-mining
operations to agricultural lands in the region of Cam Pha. In
addition, our results imply a sustained absorption of metals by
native rice plant varieties, which may lead to metal accumulation
(e.g., Cd) in human organs and in turn to severe disease.